test: Always use paths without symlinks

test: Always use paths without symlinks

When notmuch sources are at a symlinked path, some tests fail because
one part of the test uses physical path and another uses logical
path (with symlinks). For example the following test output is
produced when the test is started from /home/src/symlink-to-notmuch,
which is a symlink to /home/src/notmuch.

    FAIL   notmuch-fcc-dirs set to a string
        --- T310-emacs.26.OUTPUT        2015-12-29 08:54:29.055878637 +0000
        +++ T310-emacs.26.EXPECTED      2015-12-29 08:54:29.055878637 +0000
        @@ -1,5 +1,5 @@
         From: Notmuch Test Suite <test_suite@notmuchmail.org>
         To:
         Subject:
        -Fcc: /home/src/notmuch/test/tmp.T310-emacs/mail/sent-string
        +Fcc: /home/src/symlink-to-notmuch/test/tmp.T310-emacs/mail/sent-string
         --text follows this line--
    nil

This commit makes all paths in test scripts physical. With it, all
tests pass even when run from a symlinked directory.